Gas sensor, in particular a lambda probe, and method for production thereof

ABSTRACT

The ceramic sensor body, held inside a housing by a sealing packing, is provided with a circumferential coating of electrically insulating material in the region of the sealing packing, so that the sensor body remains potential-free with respect to the housing, even when a sealing packing is used that is made of a material having poor electrical insulating qualities. In this manner, it is also possible to use a glass putty for the sealing packing, which, while having comparatively poor insulating characteristics, provides a good sealing effect at high loading capacity.

BACKGROUND INFORMATION

[0001] The present invention relates to gas sensors, especially lambda sensors, which include a ceramic sensor body and an associated housing. A sealing packing, surrounding the sensor body at a midsection and supporting it, subdivides the housing into a measuring gas chamber, acted upon by a measuring gas, and a reference-gas chamber, able to be acted upon by a reference gas, especially by air of the atmosphere.

[0002] Today, exhaust systems of modern internal combustion engines, particularly for motor vehicles, are regularly provided with catalytic converters for converting harmful exhaust gases into harmless reaction products. In order for the catalytic converters to function well, it is necessary to feed air and fuel to the engine in a predefined proportion. The engine controls provided for this purpose are connected on their input side to a so-called lambda probe whose signals represent the composition of the exhaust gas and thus enable the engine control to adjust the ratio of fuel and combustion air in a manner optimal for the catalytic converter.

[0003] Modern heating devices in which fuels are burned to generate heat work similarly.

[0004] The sealing packing mentioned in the introduction has a plurality of functions. On the one hand, in the housing of the sensor body, it separates a measuring gas chamber or an exhaust-gas chamber from a reference-gas chamber. On the other hand, it also supports the sensor body in the housing, whose inner cross-section regularly has a shape that deviates from the outer cross-section of the sensor body. Finally, the sealing packing is to keep the sensor body electrically potential-free, or to insulate it electrically from the housing, which is generally made of metal.

[0005] Normally, the sealing packing is built up from a plurality of layers, the different layers being able to assume various main functions, such as protecting the remaining layers from pulsations of the exhaust gas, firmly supporting the sensor body in the housing, and sealingly separating the exhaust-gas chamber and the reference-gas chamber from one another.

[0006] Until now, a steatite-pressure packing into which a boron nitride disk is inserted, generally provided the sealing separation of the measuring gas chamber or exhaust gas chamber and the reference gas chamber with respect to one another. On the one hand, this ensures good electric insulation between the sensor body and the associated housing and, on the other hand, good sealing from gasoline vapors or fuel vapors as well as liquid fuel.

[0007] However, the required manufacturing effort is relatively high.

SUMMARY OF THE INVENTION

[0008] According to the present invention, the sensor body is provided with a coating of an electrically insulating material in the region of the sealing packing or inside the sealing packing, so that even materials having poor insulating properties may be used as sealing packing for the purpose of separating the exhaust gas chamber and the reference gas chamber from one another.

[0009] The present invention is based on the general concept of keeping the sensor body potential-free with respect to the electrically by an electrically insulating layer located on the sensor body, or of dispensing with the requirement of a sealing packing that has good electrical insulating properties and, in this way, offering greater constructive freedom in the choice of materials for the sealing packing.

[0010] In doing so, one utilizes the realization that the insulating layer on the sensor body in the region of the sealing packing may be produced simultaneously with the electrically insulating coatings of the sensor body at its electrodes; since these are required anyway, practically no additional production effort is required.

[0011] It is particularly advantageous that a glass putty may readily be used as a result of the present invention, which, at comparatively much lower cost, provides at least the same, but normally considerably better sealing than boron nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Exemplary embodiments of lambda sensors according to the present invention as well as their production are explained in more detail hereinafter, with the aid of the drawings.

[0013] The figures show:

[0014]FIG. 1 a schematized longitudinal section of a sensor body and the associated housing as well as the sealing packing;

[0015]FIG. 2 a sectional view according to cut line II-II in FIG. 1;

[0016]FIG. 3 exemplary production steps for the insulation coating of the sensor bodies in the region for the sealing packing;

[0017]FIG. 4 an additional possibility of producing this insulation layer.

[0018] According to FIGS. 1 and 2, a sensor body 2, in the form of a ceramic laminate, is securely accommodated in an essentially tubular housing 1 and supported in housing 1 by a sealing packing 3. Sealing packing 3 divides the interior of housing 1 into a reference gas chamber 1′, as well as an exhaust gas chamber or measuring gas chamber 1″, associated end region sensor body 2 extending into both chambers.

[0019] At its end assigned to reference gas chamber 1′, housing 1 is open, so that electrical connection lines (not shown) are able to be connected to electrodes at the sensor body. The part of housing 1 surround the exhaust gas chamber is provided with one or a plurality of smaller openings, via which the exhaust gas chamber or measuring gas chamber 1″ communicates with the exterior of housing 1. By this housing section, housing 1 projects into an exhaust branch of an internal combustion engine or heating device, sealing packing 3 sealing the exhaust branch with respect to the atmosphere.

[0020] According to the present invention, sensor body 2, in region of sealing packing 3, includes a circumferential layer 4 of electrically insulating material. This enables sensor body 1 to be kept potential-free even with respect to housing 1, which is normally made of metal, if the material of sealing packing 3 is electrically conductive to a more or less pronounced degree, or has only poor electrical insulating properties.

[0021] As a result of electrically insulating circumferential layer 4, sealing packing 3, completely or in part, may be in the form of glass putty, whose electrical insulating resistance drops to approx. 200 kΩ when the temperature of the housing rises to approximately 530 degrees C., analogously to the temperature in the exhaust branch.

[0022] It is advantageous that such a glass putty provides excellent sealing from fuel in liquid or vapor form, especially from gasoline. If required, an additional thin sealing layer or foil may also be provided within the glass putty, made of boron nitride, for example. Such a thin sealing layer or sealing foil is considerably less expensive than the thick boron nitride layer that has generally been used until now.

[0023] To improve the sealing between sealing packing 3 and sensor body 2, a sealing layer 5, made of metal, for example, may also be provided on the electrically insulating circumferential layer 4 of the sensor body.

[0024] An advantageous possibility of producing the electrically insulating peripheral layer 4 is explained with the aid of FIG. 3.

[0025] To manufacture a great number of sensor bodies 2, a large-surfaced, patterned ceramic laminate 20 is produced, which may be divided into many sensor bodies 2 by cutting along cutting lines 6.

[0026] In the area of circumferential layers 4 to be produced, slits 7, which penetrate the aforementioned laminate from its upper side to its lower side, are punched out in the large-surfaced laminate. Subsequently, a strip 8 made of an electrically insulating material may be applied using printing technology, this material also penetrating into slits 7 or being introduced into slits 7 by underpressure or similar means. Then, the large-surfaced ceramic laminate is separated along cutting lines 6, the insulation material that has penetrated into slits 7 being cut through. As a result, each sensor body 2, while forming an electrically insulating circumferential layer 4, is covered by insulating material both on its upper as well as its lower side and along its longitudinal edges.

[0027] This is followed by a sintering process during which sensor bodies 2, which have been cut out from laminate 20, are hardened together with the insulating material.

[0028] Basically, it is also possible to already separate sensor bodies 2 from one another at cutting lines 6 when slits 7 are stamped out. During the subsequent application of the insulating material, sensor bodies 2, having been separated from one another, must then be held in position by appropriate measures or tools.

[0029] The application of strips 8 made of electrically insulating material may take place simultaneously with the application of electrically insulating layers (not shown) at additional areas of the large-surfaced laminate, or of sensor bodies 2, for example, at electrode surfaces on sensor bodies 2.

[0030] To produce the electrically insulating layer 4, it is basically also possible to wrap an electrically insulating tape 9 around the sensor bodies according to FIG. 4. In its place, a transfer tape carrying the insulation material may be used, which, as a result of a subsequent high-temperature process, is dissolved or destroyed on the respective sensor body 2 during simultaneous sintering of the insulation material.

[0031] To produce sealing layer 5, it is possible to use methods that are basically equivalent to those used to produce electrically insulating circumferential layer 4. 

What is claimed is:
 1. A gas sensor, especially a lambda sensor, comprising a ceramic sensor body (2) as well as an associated housing (1) in which a sealing packing (3) is located, which surrounds and/or supports the sensor body (2), wherein the sensor body (2), in the region of the sealing packing (3), is provided with a coating (4) made of an electrically insulating material.
 2. The gas sensor as recited in claim 1, wherein the sensor body (2) is positioned at its sensitive region in a measuring gas chamber (1″) acted upon by the measuring gas, and, in an area (1′) of the housing (1) facing away from the measuring gas chamber (1″), it is separated by the sealing packing (3) in a gastight manner; and the sealing packing (3) is made of a glass putty or includes such a glass putty.
 3. The gas sensor as recited in claim 1 or 2, wherein the sealing packing (3) includes a layer or foil of boron nitride, which extends transversely to the longitudinal direction of the sensor body (2).
 4. The gas sensor as recited in one of claims 1 through 3, wherein a sealing layer or metal layer (5) is located on the coating (4) made of an electrically insulating material.
 5. A method for manufacturing gas sensors as recited in one of claims 1 through 4, wherein, to produce the sensor bodies (2), a flat laminate (20) is produced that may be separated into the sensor bodies at the cutting lines (6); and that in this laminate, in areas provided for the coating (4) with electrically insulating material, slits (7) penetrating the laminate are produced, or stamped out, along the cutting lines (6), and these are filled with the electrically insulating material during, or by the application of electrically insulating material on the upper and/or the lower side of the laminate within a strip (8) which covers the slits (7) in a transverse direction.
 6. The method as recited in claim 5, wherein the flat laminate (20) is separated at the cutting lines (6) prior to a sintering process.
 7. The method as recited in claim 6, wherein the flat laminate (20) is separated at the cutting lines (6) after application of the electrically insulating material.
 8. The method as recited in one of claims 5 through 7, wherein the sensor bodies are sintered with the electrically insulating material. 